Flexible membranes for a polishing head

ABSTRACT

A flexible membrane for a polishing head includes a main part having a first surface configured to make contact with a surface of a substrate and a second surface opposite to the first surface, a plurality of extensions extending substantially perpendicularly from the second surface of the main part so as to define a plurality of independent spaces, respectively, and an inner ring coupled with an inner side of an outermost extension of the extensions to support an edge portion of the main part. A bottom surface of the inner ring is substantially flat.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. 10-2012-0054941 filed on May 23, 2012, the entire disclosure of which is hereby incorporated by reference herein in its entirety.

1. TECHNICAL FIELD

Example embodiments relate to flexible membranes for a polishing head. More particularly, example embodiments relate to flexible membranes for a polishing head of a chemical mechanical polishing apparatus.

2. DISCUSSION OF THE RELATED ART

Chemical mechanical polishing is one of methods for a planarization process. Chemical mechanical polishing may be performed by, for example, mounting a substrate on surface of a membrane for a polishing head, forcibly bringing the substrate into contact with a rotating polishing pad, and feeding polishing slurry between the polishing pad and the substrate, so as to polish thin films formed on the substrate. In a chemical mechanical polishing method for uniformly polishing thin films of a substrate, a membrane of a polishing head on which a substrate is to be held may, for example, be divided into zones, and the pressures of the zones may be independently adjusted during a polishing process. Even if such a method is used, it may be difficult to uniformly polish regions of a large substrate.

SUMMARY

Example embodiments provide a flexible membrane for a polishing head, and the flexible membrane provides desired polishing profiles to be obtained.

According to an example embodiment, there is provided a flexible membrane for a polishing head. The flexible membrane includes a main part having a first surface configured to make contact with a surface of a substrate and a second surface opposite to the first surface, a plurality of extensions extending substantially perpendicularly from the second surface of the main part so as to define a plurality of independent spaces, respectively, and an inner ring coupled with an inner side of an outermost extension of the extensions to support an edge portion of the main part. A bottom surface of the inner ring is substantially flat.

In an example embodiment, the main part and the extensions may include a flexible material.

In an example embodiment, the inner ring may include one of a flexible material or a nonflexible material.

In an example embodiment, the inner ring may have a sidewall having a shape corresponding to a shape of a sidewall of the outermost extension, and the sidewall of the inner ring may be disposed in the sidewall of the outermost extension.

In an example embodiment, the inner ring may have a same height along a circumference of the ring.

In an example embodiment, an inner wall of the inner ring may be vertical.

In example embodiments, an inner wall of the inner ring may include a first portion having a vertical slope, a second portion having a non-vertical slope and a third portion having a vertical slope in an upward direction.

In an example embodiment, the extensions may be ring-shaped, and the spaces defined by the extensions may be concentric.

In an example embodiment, an edge-portion space defined by the extensions may be divided into a first zone and a second zone under the first zone.

In an example embodiment, the second zone may face a portion defined from an edge of the main part to a point inwardly spaced from the edge of the main part by about 10% to about 20% of the radius of the main part.

In an example embodiment, the spaces defined by the extensions may be configured to be one of independently pressurized by air or independently evacuated to a vacuum pressure.

In an example embodiment, the main part may be disk-shaped.

In an example embodiment, a top surface of the inner ring may be substantially parallel with the substrate.

In accordance with an exemplary embodiment, a flexible membrane for a polishing head, the flexible membrane is provided. The flexible membrane includes a main part having a first surface configured to make contact with a surface of a substrate and a second surface opposite to the first surface, a plurality of extensions extending substantially perpendicularly from the second surface of the main part so as to define a first space, a second space, a third space, a fourth space, and a fifth space in the membrane which are isolated from each other, and the second space is disposed vertically under the first space and the first and second spaces face an edge portion of the substrate. The third space, the fourth space and the fifth space are disposed in a direction from the second space to a center of the main part and the second space, the third space, the fourth space and the fifth space are configured to directly face the substrate. The flexible membrane further includes an inner ring coupled with an inner side of an outermost extension of the extensions to support an edge portion of the main part, wherein a bottom surface of the inner ring being substantially flat and an inner wall of the inner ring is vertical.

In example embodiments, an inner wall of the inner ring may include a portion having a vertical slope.

In accordance with an exemplary embodiment, a carrier head for a chemical polishing apparatus is provided. The carrier head includes a housing, a base assembly disposed under the housing and vertically movable, a loading chamber disposed between the housing and the base assembly and configured to apply a load to the base assembly and adjust a vertical position of the base assembly, a retainer ring configured to hold a substrate clamped to the base assembly, and a flexible membrane. The flexible member includes a main part having a first surface configured to make contact with a surface of the substrate and a second surface opposite to the first surface, a plurality of extensions extending substantially perpendicularly from the second surface of the main part so as to define a plurality of independent spaces, respectively and an inner ring coupled with an inner side of an outermost extension of the extensions to support an edge portion of the main part, and a bottom surface of the inner ring is substantially flat.

If a chemical mechanical polishing process is performed using the membrane of example embodiments, a thin film may have a uniform polished profile, and particularly, a thin film formed on an edge portion of a substrate may be uniformly polished. Therefore, defects from a chemical mechanical polishing process may be reduced. As a result, the manufacturing yield of semiconductor devices may be effectively increased.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments can be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. FIGS. 1 to 9 represent non-limiting, example embodiments as described herein.

FIG. 1 is a cross-sectional view illustrating a carrier head in accordance with an example embodiment;

FIG. 2 is a cross-sectional view illustrating a flexible membrane in accordance with an example embodiment;

FIG. 3 is a cross-sectional view illustrating the flexible membrane of FIG. 2 mounted on a polishing head;

FIG. 4 is a cross-sectional view illustrating the bottom shape of the membrane of FIG. 2 in a polishing process;

FIG. 5 is a plan view illustrating the positions of zones of the membrane of FIG. 2;

FIG. 6 is a cut-away view illustrating an inner ring of the membrane of FIG. 2;

FIG. 7 is an enlarged cross-sectional view illustrating an inner ring portion of FIG. 2;

FIG. 8 is an enlarged cross-sectional view illustrating a comparison sample inner ring for comparison with an inner ring of an example embodiment;

FIG. 9 is a cut-away view illustrating an inner ring in accordance with an example embodiment.

FIG. 10 is an enlarged cross-sectional view illustrating a membrane being coupled to the inner ring of FIG. 9.

FIG. 11 is a graph showing the results of polishing using a membrane of an example embodiment and using a membrane including the comparison sample inner ring.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments will be described more fully hereinafter with reference to the accompanying drawings, in which some example embodiments are shown. In the drawings, the sizes and relative sizes of layers and regions may be exaggerated for clarity.

It will be understood that when an element or layer is referred to as being “on,” “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. Like numerals refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present inventive concept. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof

FIG. 1 is a cross-sectional view illustrating a carrier head in accordance with an example embodiment.

Referring to FIG. 1, the carrier head 100 may be included in, for example, a chemical mechanical polishing apparatus so as to put a substrate onto a polishing pad by pressure for polishing the substrate. The carrier head 100 may include, for example, a housing 102, a base assembly 104, a loading chamber 108, a retainer ring 200, a carrier ring 400, and a membrane 500 defining a plurality of compressible chambers.

For example, the housing 102 may have a circular shape and be connected to a driving shaft so as to be rotated during a polishing process. A passage (not shown) for controlling the pneumatic pressure of the carrier head 100 may be formed through the housing 102. The base assembly 104 may be disposed under the housing 102 and be, for example, vertically movable. The loading chamber 108 may be disposed between the housing 102 and the base assembly 104 to apply a load to the base assembly 104 and adjust the vertical position of the base assembly 104.

A substrate 10 may be held by, for example, the retainer ring 200 clamped to the base assembly 104.

The membrane 500 may, for example, be flexible and have a main part 502 on which a substrate may be mounted. The membrane 500 includes, for example, extensions 504 extending perpendicularly from the main part 502, and an inner ring 506 coupled to an inner side of an outermost extension 504 a. The membrane 500 will be described later in more detail.

FIG. 2 is a sectional view illustrating the flexible membrane in accordance with an example embodiment. FIG. 3 is a sectional view illustrating the flexible membrane of FIG. 2 when the flexible membrane is mounted on a polishing head. FIG. 4 is a sectional view illustrating the bottom shape of the membrane of FIG. 2 in a polishing process. FIG. 5 is a plan view illustrating the positions of zones of the membrane of FIG. 2.

Referring to FIGS. 2 and 3, the flexible membrane 500 includes, for example, a disk-shaped main part 502 configured to be brought into contact with a side of a substrate 10, extensions 504 extending perpendicularly from the main part 502, and an inner ring 506 coupled to an inner side of an outermost extension 504 a.

The main part 502 and the extensions 504 of the membrane 500 may include, for example, flexible and elastic materials. Examples of the materials that may be used for the main part 502 and the extensions 504 of the membrane 500 may include but are not limited to chloroprene, ethylene propylene, silicone, ethylene propylene diene M-class rubber, silicone rubber, etc. These can be used alone or in a combination thereof. The main part 502 and the extensions 504 of the membrane 500 may be formed, for example, in one body by using the same material. The membrane 500 may have properties such as, for example, being hydrophobic, durable, and chemically inert to be used in a polishing process. The main part 502 may have a diameter, for example, equal to or slightly smaller or larger than that of the substrate 10 so that the substrate 10 may be accommodated on the main part 502.

The main part 502 may have, for example, a first surface as a substrate mounting surface for making contact with a substrate 10. The extensions 504 may be provided on a second surface of the main part 502 opposite to the first surface.

Each of the extensions 504 may have, for example, a protruded ring shape. Therefore, spaces may be defined on the second surface of the main part 502 by, for example, the extensions 504, and the spaces may be, for example, concentric. The number of the spaces may be determined by, for example, the number of the extensions 504.

For example, in an example embodiment, five spaces may be defined by the extensions 504. In the following description, the spaces defined by the extensions 504 will be referred to as first to five zones Z1 to Z5 in the direction from the edge to the center of the disk-shaped main part 502.

As shown in FIGS. 2 to 5, the first zone Z1 may face, for example, an edge portion of the substrate 10.

The second zone Z2 may, for example, be positioned vertically under the first zone Z1. That is, the first and second zones Z1 and Z2 may, for example, face the edge portion of the substrate 10. The second zone Z2 faces a portion defined from the edge of the main part 502 to a point inwardly spaced from the edge of the main part 502 by, for example, about 10% to about 20% of the radius of the main part 502. For example, if a substrate 10 having a diameter of about 300 mm is polished, the second zone Z2 may be a ring-shaped space defined from a position spaced about 120 mm to about 135 mm from the center of the main part 502 to the edge of the substrate 10 (about 150 mm from the center of the main part 502).

The third, fourth, and fifth zones Z3, Z4, and Z5 may be arranged, for example, in a direction from the second zone Z2 to the center of the main part 502.

For example, the first zone Z1 may be an auxiliary space to compensate for a polishing rate at the edge portion of the substrate 10, and the second to fifth zones Z2 to Z5 may directly face the substrate 10.

As shown in FIG. 1, when a head assembly is completely assembled by mounting the membrane 500 on a head, upper sides of the first to fifth zones Z1 to Z5 defined in the membrane 500 by the extensions 504 may, for example, be isolated to separate the first to fifth zones Z1 to Z5 from one another.

When air is injected in the isolated first to fifth zones Z1 to Z5, the first surface of the membrane 500 may be expanded toward the substrate 10. As the first surface of the membrane 500 may be expanded, the substrate 10 facing the first to fifth zones Z1 to Z5 may be pressured. That is, for example, four regions of the substrate 10 may be pressured independently by adjusting each air pressure injected in the first to fifth zones Z1 to Z5.

The first to fifth zones Z1 to Z5 may be used as vacuum spaces as well as being used as spaces for applying pressure. That is, if the first to fifth zones Z1 to Z5 are evacuated to a vacuum pressure, the first surface of the membrane 500 may be deformed in a head direction opposite to the substrate 10. Therefore, the substrate 10 may be kept in a state where the substrate is attached to the first surface of the membrane 500.

As described above, the extensions 504 function as boundaries between neighboring zones. Thus, the inner shapes of the first to fifth zones Z1 to Z5 may be varied according to the shapes of the extensions 504. For example, as the widths of the extensions 504 increase, the inside spaces of the first to fifth zones Z1 to Z5 may be narrowed, and as the widths of the extensions 504 decrease, the inside spaces of the first to fifth zones Z1 to Z5 may be widened. Therefore, the shapes of the extensions 504 may be adjusted so that the inside spaces of the first to fifth zones Z1 to Z5 may be properly pressurized.

The second zone Z2 of the membrane 500 facing the edge portion of the substrate 10 may have, for example, boundary conditions different from those of the neighboring third to fifth zones Z3 to Z5.

For example, each of the third to fifth zones Z3 to Z5 may have one extension. As shown in. FIG. 5, if air pressure is applied to the third to fifth zones Z3 to Z5, the flexible main part 502 and the extensions 504 defining the third to fifth zones Z3 to Z5 may, for example, expand in radial directions. Therefore, forces may be applied to the neighboring extensions 504 in different directions, and thus forces acting on boundaries between the third to fifth zones Z3 to Z5 may be uniform. As a result, bottom portions of the main part 502 corresponding to the third to fifth zones Z3 to Z5 may become flat, and thus portions of the substrate 10 corresponding to the third to fifth zones Z3 to Z5 may be uniformly polished.

However, an extension disposed at a side of the second zone Z2 facing the edge portion of the substrate 10 is also used as a boundary of the third zone Z3, and an extension disposed at the other side of the second zone Z2 is not used as a boundary for another zone but faces the retainer ring 200 that is not flexible. When air pressure is applied to the inside of the second zone Z2, forces may be applied, for example, to the extension located between the second zone Z2 and the third zone Z3 in different directions, and thus the forces may be balanced. However, the outermost extension 504 a facing the retainer ring 200 is restrained from expanding laterally. When air pressure is applied to the first to fifth zones Z1 to Z5 in a polishing process, a bottom portion of the main part 502 corresponding to the second zone Z2 may be deformed in a shape different from other neighboring portions. As the bottom side of the main part 502 may be deformed non-uniformly due to non-uniform forces applied to the second zone Z2 as described above, the edge portion of the substrate 10 facing the second zone Z2 may not be uniformly polished as compared with other portions of the substrate 10. For example, an inner region of the second zone Z2 adjacent to the third zone Z3 may be affected by air pressure applied to the third zone Z3 and air pressure applied downward from the first zone Z1, and thus the substrate 10 may be rapidly polished by the inner region of the second zone Z2. However, as the outermost portion of the second zone Z2 faces the retainer ring 200, expansion of the second zone Z2 of the main part 502 may be restricted by the retainer ring 200. Therefore, when air pressure is applied to the second zone Z2, the first surface of the main part 502 may become not flat, but sloped because the main part 502 insufficiently expands at a portion facing the retainer ring 200. Then, a portion of the substrate 10 facing the second zone Z2 may not be uniformly polished.

The inner ring 506 may increase polishing uniformity at the edge portion of a substrate 10 facing the second zone Z2.

The inner ring 506 may be coupled to the outermost extension 504 a formed along the outermost side of the main part 502, so as to support the outermost extension 504 a. That is, the outermost extension 504 a located on the outermost side of the main part 502 may be fixed by the inner ring 506.

FIG. 6 is a cut-away view illustrating the inner ring 506 included in the membrane 500 shown in FIG. 2. FIG. 7 is an enlarged view illustrating an inner ring portion shown in FIG. 2.

As shown in FIGS. 2 to 7, the inner ring 506 may be coupled to a sidewall of the outermost extension 504 a (hereinafter referred to as a first extension 504 a) that faces the retainer ring 200 in the second zone Z2. The inner ring 506 may include, for example, flexible materials. Examples of the materials that may be used for the inner ring 506 may include but are not limited to chloroprene, ethylene propylene, silicone, ethylene propylene diene M-class rubber, silicone rubber, etc. These can be used alone or in a combination thereof. Alternatively, in an example embodiment, the inner ring 506 may include, for example, a nonflexible material.

A sidewall of the inner ring 506 may be configured to be coupled to the sidewall of the first extension 504 a. The sidewall of the inner ring 506 may have, for example, a shape corresponding to the shape of the sidewall of the first extension 504 a so as to be inserted in the sidewall of the first extension 504 a.

In a state where the inner ring 506 is coupled to the sidewall of the first extension 504 a, a bottom surface 506 a and a top surface 506 b of the inner ring 506 may, for example, be flat without any slope. When the inner ring 506 is coupled to the sidewall of the first extension 504 a, the bottom surface 506 a and the top surface 506 b of the inner ring 506 may, for example, be parallel with surfaces of the substrate 10. Thus, when the inner ring 506 is coupled to the sidewall of the first extension 504 a, the inner ring 506 may have, for example, the same height along the circumference thereof.

The inner volume in the lower portion of the second zone Z2 may be varied according to the shape of the inner ring 506. The bottom surface 506 a of the inner ring 506 may cover, for example, about 50% to about 95% of the top width of the second zone Z2. Thus, the lower portion of the second zone Z2 may have a volume limited by the bottom surface 506 a of the inner ring 506. As the bottom surface 506 a of the inner ring 506 is flat and parallel with the substrate 10, the inside space of the second zone Z2 facing the substrate 10 may be uniform at any position.

In a coupled state with the sidewall of the first extension 504 a, an inner wall 506 c of the inner ring 506 (a wall opposite to a mounting surface) may be, for example, vertical.

The first zone Z1 may be located on top of the second zone Z2, and a vertical force may be additionally applied to the second zone Z2 when air pressure is applied to the first zone Z1. Due to the inner ring 506 being coupled to the sidewall of the first extension 504 a as described above, the second zone Z2 facing the first zone Z1 may have almost the same volume at any position. Therefore, a force additionally applied from the first zone Z1 to the second zone Z2 may be uniformly distributed in the second zone Z2.

As a force is uniformly distributed in the second zone Z2 as described above, the edge portion of a substrate 10 corresponding to the second zone Z2 may be uniformly polished. As the edge portion of the substrate 10 can be uniformly polished, the overall polishing uniformity of the substrate 10 may be increased.

If the inner ring 506 is not used, the first extension 504 a located along the edge of the main part 502 is not supported, and thus the edge portion of the substrate 10 may not be uniformly polished and a relatively large polishing deviation may result.

If the bottom surface 506 a and the top surface 506 b of the inner ring 506 are not flat but are sloped when the inner ring 506 is coupled to the sidewall of the first extension 504 a, the height of the inner ring 506 varies according to position. In this case, the inner volume of the second zone Z2 is varied according to position, and thus polishing uniformity at the second zone Z2 may be decreased.

An exemplary case of using an inner ring different from the inner ring 506 of the example embodiments will now be described briefly for the purpose of comparison.

FIG. 8 is an enlarged sectional view illustrating a comparison sample inner ring for comparison with an inner ring of an example embodiment.

Referring to FIG. 8, when the comparison sample inner ring is installed, a sidewall 506 c of the comparison sample inner ring is sloped. Therefore, a bottom surface 506 a and a top surface 506 b of the comparison sample inner ring are not flat.

In this case, the inside space of the second zone Z2 may not be uniform but have a relatively wide region and a narrow region. In addition, an upper portion of the second zone Z2 facing the first zone Z1 may be varied in volume. Therefore, a force additionally applied from the first zone Z1 to the second zone Z2 may not be uniformly distributed in the second zone Z2. When the comparison sample inner ring is used, a relatively excessive force may be applied to a wide region of the second zone Z2, and the polishing rate of a substrate may be reduced progressively toward the edge of the substrate.

FIG. 9 is a cut-away view illustrating an inner ring in accordance with an example embodiment. FIG. 10 is an enlarged cross-sectional view illustrating a membrane being coupled to the inner ring of FIG. 9.

As shown in FIGS. 9 and 10, a first sidewall 522 of the inner ring 520 may be configured to be coupled to the sidewall of the first extension 504 a. The first sidewall 522 of the inner ring 520 may have, for example, a shape corresponding to the shape of the sidewall of the first extension 504 a so as to be inserted in the sidewall of the first extension 504 a.

In a state where the inner ring 520 is coupled to the sidewall of the first extension 504 a, a bottom surface 524 of the inner ring 520 may, for example, have a flat shape without any slope. When the inner ring 520 is coupled to the sidewall of the first extension 504 a, the bottom surface 524 of the inner ring 520 may, for example, be parallel with surfaces of a substrate for polishing.

In a coupled state with the sidewall of the first extension 504 a, a second side wall of the inner ring 520 (a side wall opposite to the first sidewall 522) may have a first portion 526 a having a vertical slope, a second portion 526 b having a non-vertical slope and a third portion 526 c having a vertical slope in an upward direction.

In a state where the inner ring 520 is coupled to the sidewall of the first extension 504 a, a width of the inner ring may be narrowed toward an upper portion by the slope of the second portion 526 b. An acute angle of the slope of the second portion 526 b with the substrate may be about 30 to about 60°.

When the inner ring 520 of the FIGS. 9 and 10 is coupled to a membrane, the bottom surface 524 of the inner ring 520 is flat and is parallel with the substrate 10. Therefore, an inner space of the second zone Z2 facing the substrate 10 may be uniform at any position. As a force is uniformly distributed in the second zone Z2 as described above, the edge portion of a substrate corresponding to the second zone Z2 may be uniformly polished. As the edge portion of the substrate can be uniformly polished, the overall polishing uniformity of the substrate may be enhanced.

Comparison Experiment

A thin film was polished by using a membrane including an inner ring in accordance with an example embodiment. For the purpose of comparison, a thin film was polished by using a membrane including the comparison sample inner ring shown in FIG. 8.

The other parts of a polishing apparatus were not changed except for the inner rings. In addition, the polishing conditions were used. Substrates having a diameter of about 300 mm were polished.

FIG. 11 is a graph showing the results of polishing using a membrane in accordance with an example embodiment and using a membrane including the comparison sample inner ring.

In FIG. 11, the x-axis denotes a radial distance measured from the center of a substrate, and the y-axis denotes a thickness removed by polishing. That is, FIG. 11 shows the thickness of a thin film removed by polishing in an annular region of a substrate defined from about 130 mm to about 150 mm (edge of the substrate) in radius.

Referring to FIG. 11, when the member of an example embodiment (denoted by reference numeral 50) was used, the thickness of the thin film removed by polishing in the annular region was relatively uniform. However, when the membrane including the comparison sample inner ring (denoted by reference numeral 52) was used, the thickness of the thin film removed by polishing in the annular region was not uniform with a relatively large deviation.

The flexible membrane of an example embodiment provided for a polishing head may be used in a polishing apparatus that is used for polishing semiconductor devices.

Having described exemplary embodiments of the present invention, it is further noted that it is readily apparent to those of ordinary skill in the art that various modifications may be made without departing from the spirit and scope of the invention which is defined by the metes and bounds of the appended claims. 

What is claimed is:
 1. A flexible membrane for a polishing head, the flexible membrane comprising: a main part having a first surface configured to make contact with a surface of a substrate and a second surface opposite to the first surface; a plurality of extensions extending substantially perpendicularly from the second surface of the main part so as to define a plurality of independent spaces, respectively; and an inner ring coupled with an inner side of an outermost extension of the extensions to support an edge portion of the main part, wherein a bottom surface of the inner ring being substantially flat.
 2. The flexible membrane of claim 1, wherein the main part and the extensions comprise a flexible material.
 3. The flexible membrane of claim 1, wherein the inner ring comprises one of a flexible material or a nonflexible material.
 4. The flexible membrane of claim 1, wherein the inner ring has a sidewall having a shape corresponding to a shape of a sidewall of the outermost extension, and the sidewall of the inner ring is disposed in the sidewall of the outermost extension.
 5. The flexible membrane of claim 1, wherein the inner ring has a same height along a circumference of the inner ring.
 6. The flexible membrane of claim 1, wherein an inner wall of the inner ring is vertical.
 7. The flexible membrane of claim 1, wherein an inner wall of the inner ring includes a first portion having a vertical slope, a second portion having a non-vertical slope and a third portion having a vertical slope in an upward direction.
 8. The flexible membrane of claim 1, wherein the extensions are ring-shaped, and the spaces defined by the extensions are concentric.
 9. The flexible membrane of claim 1, wherein an edge-portion space defined by the extensions is divided into a first zone and a second zone under the first zone.
 10. The flexible membrane of claim 9, wherein the second zone faces a portion defined from an edge of the main part to a point inwardly spaced from the edge of the main part by about 10% to about 20% of a radius of the main part.
 11. The flexible membrane of claim 1, wherein the spaces defined by the extensions are configured to be one of independently pressurized by air or independently evacuated to a vacuum pressure.
 12. The flexible membrane of claim 1, wherein the main part is disk-shaped.
 13. The flexible membrane of claim 1, wherein a top surface of the inner ring is substantially parallel with the substrate.
 14. A flexible membrane for a polishing head, the flexible membrane comprising: a main part having a first surface configured to make contact with a surface of a substrate and a second surface opposite to the first surface; a plurality of extensions extending substantially perpendicularly from the second surface of the main part so as to define a first space, a second space, a third space, a fourth space, and a fifth space in the membrane which are isolated from each other, wherein the second space is disposed vertically under the first space and the first and second spaces face an edge portion of the substrate, wherein the third space, the fourth space and the fifth space are disposed in a direction from the second space to a center of the main part and wherein the second space, the third space, the fourth space and the fifth space are configured to directly face the substrate; and an inner ring coupled with an inner side of an outermost extension of the extensions to support an edge portion of the main part, wherein a bottom surface of the inner ring being substantially flat and an inner wall of the inner ring is vertical.
 15. The flexible membrane of claim 14, wherein each of the extensions have a protruded ring shape.
 16. The flexible membrane of claim 14, wherein the main part and the extensions are formed in one body and are formed of a same material as each other.
 17. The flexible membrane of claim 14, wherein an inner volume of a lower portion of the second zone is limited by the bottom surface of the inner ring.
 18. The flexible membrane of claim 17, wherein the bottom surface of the inner ring covers about 50% to about 95% of a top width of the second space.
 19. The flexible membrane of claim 12, wherein a top surface of the inner ring is substantially parallel with the substrate.
 20. The flexible membrane of claim 12, wherein an inner wall of the inner ring includes a portion having a vertical slope. 